This invention relates to the area of oil and natural gas exploration and, more particularly, to a method for identifying zones of geological formations having significant water saturations in which the water is essentially or entirely bound or immobile and from which any hydrocarbons present may be recovered without significant attendant water.
Subsurface reservoirs of natural gas and/or petroleum, hereinafter referred to generically as "hydrocarbons" are typically found trapped in permeable geological strata beneath a layer of impermeable strata material. A hydrocarbon will "float" upon any ground water present although typically, a transition zone will exist between the two fluids due to the water being raised by capillary action of the permeable strata material. In some regions, impermeable layers may be relatively closely stacked atop one another trapping thin zones of what may be essentially hydrocarbons, essentially water or mixed hydrocarbons and water. A well bore dropped through the formation and various layers may produce water if tapped in a transition region or mixed hydrocarbon and water zone. The cost of transporting, separating and disposing of the attendant water adds sufficiently to production costs that hydrocarbon reservoirs have often been left untapped where it is expected or believed they would produce excessive amount of attendant water.
The determination of the location and amount of ground water present at various levels of a formation is typically based upon the interpretation of conventional electrical (i.e., a resistivity) logs taken through a borehole dropped through the formation. Water saturation of the available pore space of the formation is determined from the resistivity log measurements using the Archie equation: EQU S.sub.w.sup.n =a.multidot.R.sub.w /.phi..sup.m .multidot.R.sub.t ( 1)
where "S.sub.w " is the water saturation as a fraction of the available pore space of the formation, "a" is a formation resistivity co-efficient, "R.sub.w " is the formation water resistivity, ".phi." is the fractional formation porosity, "R.sub.t " is the formation resistivity indicated by the resistivity log, "n" is the saturation exponent and "m" is the porosity or cementation exponent. The Archie equation may be expressed in other ways and there are numerous methods in the art for determining, measuring or otherwise obtaining the various components needed to predict water saturation S.sub.w from the log-indicated resistivity, R.sub.t, using the equation in any of its forms.
It is widely recognized that a certain portion of the ground water remains essentially immovably bound to the formation rock because of capillary action and surface tension. This water is at various times also referred to as the "immobile", "residual" or "irreducible" water saturation of the formation and is expressed as a percentage of the pore space of the formation. However, lacking a tool by which to determine the extent to which the log indicated water was immovably bound or free and based upon the long term experience, it has been common practice in the oil industry to leave untapped hydrocarbon reservoirs having significant resistivity log indicated water saturations (i.e. water saturations of about 50% or more of the formation pore space). However, hydrocarbons have been produced, on occasion, with little or no attendant water from so-called "low-resistivity" formations where conventional resistivity logs have indicated water saturations in excess of 50% and, at times, even in excess of 80% of the formation pore space. With the rising value of petroleum and natural gas, it is becoming increasingly important to be able to identify all potentially recoverable hydrocarbon reservoirs, including those located in the "low-resistivity" formations which have these significant immobile water saturations.
One method to determine immobile water saturations is to measure them, indirectly, by means of a Schlumberger nuclear magnetism logging tool or comparable logging device. As described by Herrick et al. in a paper entitled "An Improved Nuclear Magnetism Logging System and its Application to Formation Evaluation" presented at the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, Sept. 23-26, 1979, this device measures by nuclear magnetic resonance the bulk or movable hydrogen bearing molecules of the formation. A component of the Schlumberger device which is passed through a borehole into a formation creates a strong local magnetic field essentially perpendicular to the earth's magnetic field and polarizes nearby hydrogen nuclei. After allowing sufficient time for the hydrogen nuclei to equilibrate in the polarized state the local magnetic field is rapidly terminated. The polarized hydrogen nuclei precess under the influence of the earth's magnetic field generating a characteristic signal decaying with time known as a free induction decay ("FID") signal. The decay time of the FID signal varies with the state of the molecules having the hydrogen nucleus or nuclei. The decay times of solid materials are shorter than those of bound fluids which themselves are shorter than free or bulk fluids. The free induction decay signal detected in situ by the nuclear magnetic log ("NML") is in reality a multiplicity of signals generated by the various minerals and fluids having hydrogen nuclei, some of the fluids being bound by varying degrees of surface tension and capillary action. From these measurements the NML determines the amount of free fluid ("Free Fluid Index" or "free fluid porosity") in a geological formation, the remainder of the pores space being assumed to be occupied by bound fluid. An arbitrary cut-off time is used in the NML to distinguish between (i.e. bulk free) hydrogen containing fluids and other sources of hydrogen producing an FID signal. Thus the NML does not measure bound water or oil. Moreover it cannot distinguish between water and certain types of light hydrocarbons. It further requires conditioning of the borehole mud column with a magnetite slurry before logging can be undertaken to prevent interference of the borehole fluid with the measuring process, an added expense and significantly more expensive and time consuming if not performed before circulation of the borehole mud is stopped. It would be desirable to provide other methods of determining irreducible water saturations using tools and methods more typically applied when drilling and investigating boreholes.
Many researchers in the field believe that irreducible water saturation is related in some way to other, measurable characteristics of the formation rocks. For example, some have noted an apparent correlation between formation rock surface areas and irreducible water saturations. See Murphy and Owens, "A New Approach for Low-Resistivity Sand Log Analysis", JOUR. OF PETR. TECH., pp. 1302-1306, November, 1972. Murphy and Owns measured the surface areas of core samples obtained from a number of widely scattered North and South American sites using a nitrogen absorption method and the minimum interstitial water saturations by porous-disc capillary pressure method. By their measuring techniques, the irreducible water saturation-surface area relationships varied from site to site and thus they concluded could not be represented by a general relationship. They hypothesized that these differences arose under the influence of such secondary factors as the presence of varying amounts and types of clay, the presence of ashy materials and different pore geometries in the rock. They proposed that a surface area/irreducible water saturation relationship be determined from measurements of both characteristics in a number of core samples obtained from a given site and that the surface area, irreducible water saturation or both be further correlated to a so-called "shaliness factor" determined from the response of a gamma ray log, SP log, or density and velocity logs. Irreducible water saturations throughout the formation could thereafter be predicted for the formation from its log-indicated shaliness factor. Murphy and Owens further noted that these predicted irreducible water saturations could be compared with water saturations determined from the response of a resistivity log using the aforesaid Archie relationship. If the log-indicated water saturation had the same or nearly the same or was less than the value of the predicted, irreducible water saturation along an interval, that interval was identified as potential water-free or low water cut hydrocarbon productive zone. However, if the irreducible water saturations were distinctly different and the log-indicated values exceeded the predicted irreducible values, the zone was identified as water productive. Timur, in an earlier article entitled, "An Investigation of Permeability Porosity, & Residual Water Saturation Relationships for Sandstone Reservoirs", THE LOG ANALYST, pp. 8-17 (July-August, 1968), also notes in a discussion of the relationship among irreducible water saturation, permeability and porosity, that others have previously assumed irreducible water saturations to be linearly related to the pore volume specific surface areas (i.e. surface area per unit volume of pore space) of the rock material. See also Guillotte et al, "Smackover Reservoir: Interpretation Case Study of Water Saturation Versus Production", XXIX TRANS-NOW GULF COAST ASSOC. OF GEOL. SOC., pp. 121-126 (1969), for further discussion of the relationship among irreducible water saturation, porosity and permeability.